1. Field of the Invention
The present invention relates to an imaging lens and an imaging system. In particular, the present invention relates to an imaging lens used in an imaging apparatus that corrects blurs in an image by performing contrast recovery processing on original image data obtained by imaging, and an imaging system using the imaging lens.
2. Description of the Related Art
Conventionally, a technique of imaging an optical image through an imaging lens that is a depth-of-field extended optical system is known. In the technique, an original image in which a subject or subjects of photography are blurred for the entire distance of photography is obtained, and contrast recovery processing is performed on the original image to eliminate blurs in the original image. Specifically, a technique of increasing the contrast of the original image, thereby obtaining an image without blurs that is obtainable by a lens having a large depth of field is known. This type of lens is called as EDOF (Extended/Extension of Depth of Field/Focus) lens.
The contrast recovery processing corrects the original image obtained through the depth-of-field extended optical system. In the contrast recovery processing, a restoration filter the properties of which are opposite to the properties of blurs of images obtained by the depth-of-field extended optical system is applied to the original image. Consequently, the contrast of each subject arranged in a wide range of photography from a short distance to a long distance is increased (for example, the outline (edge) of the image becomes sharp and clear).
It is ideal that the depth-of-field extended optical system forms an optical image of a subject or subjects on an imaging plane in such a manner that a constant blur is given to the optical image, regardless of the distance of photography to the subject or subjects.
More specifically, the depth-of-field extended optical system forms point images on the imaging plane in such a manner that each of the point images representing a subject or subjects arranged at various photography distances is blurred at a constant degree (deteriorated point images having constant light intensity distribution), regardless of the distance of photography. Further, the contrast recovery processing is performed on an image obtained by imaging an optical image composed of the deteriorated point images having constant light intensity distribution (point scatter). The contrast recovery processing is performed on the image obtained by imaging the optical image so that an image composed of target point images (for example, point images having ideal light intensity distribution) is restored. The target point images have light intensity distribution that is a restoration target value of the deteriorated point images.
According to the technique as described above, it is possible to obtain a high contrast image in the entire area of the image (the depth of field of the image is extended) for a subject at any distance of photography without reducing the aperture of the imaging lens, in other words, without reducing a receiving light amount (please refer to Japanese Unexamined Patent Publication No. 2000-123168 (Patent Document 1), U.S. Patent Application Publication No. 20090128655 (Patent Document 2), U.S. Patent Application Publication No. 20090128668 (Patent Document 3), and Japanese Patent No. 3275010 (Patent Document 4).
As the contrast recovery processing, image restoration processing by Fourier transformation, edge enhancement processing, gamma correction processing, contrast enhancement processing, or the like is known to be adoptable, for example.
However, it is difficult to realize an ideal depth-of-field extended optical system that forms an optical image representing each point (each subject) to be photographed, as point images having constant light intensity distribution, regardless of the distance of photography.
Meanwhile, it is known that the depth of field of an optical system as it looks increases, as the spherical aberration of the optical system is increased. Therefore, the spherical aberration of an optical system may be simply increased so as to increase the depth of field as it looks, and the optical system may be used as a depth-of-field extended optical system.
However, when the spherical aberration of the optical system is increased, the diameter of a point image formed on an imaging plane increases as the spherical aberration increases. When a target point image is obtained by performing contrast recovery processing on the point image having a large diameter, more noise is included in the target point image as the diameter of the point image increases. Therefore, there is a problem that the amount of noise included in a blur recovery image composed of a multiplicity of target point images obtained by contrast recovery processing becomes large.